1. Field of the Invention
The present invention relates to a nitrogen oxide sensor for detecting a concentration of a nitrogen oxide. More particularly, the invention relates to a nitrogen oxide device for converting NO into NO.sub.2 or NO.sub.2 into NO electrochemically.
2. Prior Art
A solid element type of nitrogen oxide sensor has been representatively disclosed in the Japanese Patent Laid-Open Publication No. Hei 4-142455 (1983). This sensor is equipped on an ion conductor with a nitrate electrode and a reference electrode to measure an electromotive force to be generated between the electrodes, so that it has sensitivity to NO and NO.sub.2. Since the sensitivities of the electrodes to NO and NO.sub.2 are different, however, the NOx concentration cannot be detected in an environment to be measured where the two gases (NO and NO.sub.2) coexist, or the concentration of neither NO nor NO.sub.2 can be detected.
In order to improve the sensitivities to NO and NO.sub.2, there has been proposed (in the Japanese Patent Laid-Open Publication No. Hei 6-123726(1995)) an electromotive type sensor in which an oxide catalyzer of NO is applied to or mixed with an auxiliary electrode. According to this method, the NO gas in the coexisting gas of NO and NO.sub.2 can be oxidized into a single gas of NO.sub.2 so that the NOx concentration can be detected. Like the analyzing method of the prior art, however, the accuracy is decided by the oxidizing ability of the catalyst so that the NOx concentration is different from the actual value. Moreover, these sensors are troubled by their moisture and heat resistances because they employ a nitrate in the auxiliary electrode, so that they are substantially difficult for the practical purposes from the standpoint of long-term stability.
On the other hand, there has also been reported a sensor for measuring a change in the electric conductivity, as based on the NOx concentration, by making use of semiconductor characteristics of various oxides. In the Japanese Patent Laid-Open Publication No. Hei 6-160324 (1995), for example, there has been proposed a sensor using tin oxides as a gas sensitive member. However, this sensor is also different in the sensitivity to NO and NO.sub.2 so that it cannot detect the NOx concentration in an environment to be measured where the two gases coexist.
In recent years, there has been proposed (in SAE TECHNICAL PAPER 960334 or the Japanese Patent Laid-Open Publication No. Hei 8-271476 (1997)) a method for electrolyzing NOx to detect the NOx concentration from the electrolytic current value. The detection principle itself of this sensor is developed from the electrolytic current type sensor which has been widely applied in the prior art to other gases. Specifically, an ion conductor is provided with two chambers, the first one of which lowers the oxygen concentration in the environment to be measured substantially to zero and reduces NO.sub.2 to NO by an oxygen pump and the second one of which is equipped with electrodes for ionizing the oxygen produced from the reduced NO in the measured environment by applying a voltage to the electrodes thereby to detect the NOx concentration. The NOx concentration to be detected by the sensor highly depends on the performance of the oxygen pump. When the concentration of the gas to be detected is low, the interference of the residual oxygen concentration in the measured environment is high, and the signal current is minute. In a noisy circumstance as in an automobile, the S/N ratio is so poor that the NOx concentration is difficult to detect accurately.
We have proposed a mixed potential type NOx sensor and have filed patent applications published as Japanese Patent Laid-Open Publication Nos. Hei 6-194605 (1995), Hei 6-216698 (1995) and Hei 6-216699 (1995). However, these constructions are excellent in sensitivity to NO or NO.sub.2 but are troubled by the mutual interference of NO or NO.sub.2 or by the interference of a reducing gas.